126 Proceedings of the Royal Irish Academy. 



and irregularly constituted, of orthoclase, quartz, and silvery white 

 mica," answeiing Cotta's description for tiie rock he calls peffmafi/te* 



In some places the crystal of felspar are very large, they being two 

 inches or more long. This class of rockf in places may have been 

 formed subsequent to the associated granite ; in others it has not, as 

 will hereafter be pointed out. 



Yeins of segregation ( Gratiit^te), always fine, are not uncommon, 

 and two or more systems may occur together, their different ages being- 

 known by their entering and displacing each other. 



These are more or less regidar, generally having an angular 

 arrangement, apparently occupying lines of joints, and they seem to 

 be due to matter which has segregated from the liquid or semi- 

 liquid portions near the interior of the mass having been pressed up 

 into the shrinkage fissure formed in the consolidated portion, as the 

 latter cooled, each successive system of cracks having their own 

 veins. 



^Tien looking over an explance of the Gralway-type-granite, it is 

 not unusual to find the large crystal of felspar lying in irregular 

 parallel lines, while towards the north-east and north margin of the 

 great tract that bounds Galway Bay on the north, the rock is found 

 gradually to become more and more foliated {gneissoid granite or grani- 

 toid gneiss), until eventually it graduates tloi'ough gneiss into schist 

 (see Section, Figure S, PI. 12). Towards the south, however, in the 

 vicinity of Galway Bay, another change takes place, as here the rock 

 graduates into a more or less even-gi'ained granite, in which, as a rule, 

 none of the felspar is poi-phyritically developed. This even-grained 

 granite seems to extend as coui-ses into the porphyritic granite, while 

 associated in places are many elvanytes. It may be here remarked 

 that, in the immediate neighbourhood of Galway, there are many 

 elvanytes, as a mle, in large dykes, that are associated with courses or 

 dykes of granite, all usually running with a general north and south 

 bearing. 



The conspicuous essentials of the even-grained granite are in 

 general very similar to those in the porphyritic variety, but not 

 always ; as in places, coui'ses of this granite vary not only in texture 

 and structure, but apparently also in composition : and what may 

 perhaps be more important, in one place, where gneiss overlies these 

 granites, the dip and strike of the courses seems to be similar to 

 the dip and strike of the stratification of the gneiss. In the neigh- 

 bourhood of Furbogh were observed patches of a very hornblendic 

 and titanitic granite, in which there is none of the flesh-coloured 

 orthoclase, or white mica, and only a little black mica. Toward 

 the west and north-west, the Galway-type-granite graduates into the 

 Omey -type-granite. 



* Cotta, 1. c, p. 206. 



t These veins probablv belong to the class called Endogenous by Himt. See 

 p. 131. 



